Converting Trash to SteamConverting Trash to Steam

Two150-hp regenerative crane system drive retrofits on the 10-ton overhead cranesat the Covanta Plymouth Renewable Energy plant in Conshohocken, PA, areyielding savings of more than $160,000 per year, including about $34,000 ofenergy savings alone.

Known locally as the "trash-to-steam"plant, it controls the waste flow for 24 municipalities, a total of more than 1,200tons per day, and generates more than 32 MW of renewable energy. The facilityis equipped with a water-cooled turbine generator condenser to convert steam towater for continuous cycling of water through the boilers. Two overhead cranesare used around-the-clock for handling the trash and moving it to the infeed ontwo reciprocating grate and water wall furnaces that process 608 tons per day.

These cranes were proving to be a maintenance nightmare, withconsiderable downtime and energy costs spiralling out of control. The drivesystems on the cranes wasted the excess energy created during braking of thehoist in the form of heat generated by a wound rotor resistor. But the heatalso created other problems, requiring air conditioning to bring the controlenvironment within the required 104F. In addition, the control system createdexcessive wear and tear on all components with brake pad maintenance running atsome $18,000 per year (per crane) and quarterly motor rebuilds at a maintenancecost of $48,000 each year. Almost continuous maintenance created other cost aswell because of considerable downtime for both cranes.

The company at that time, Montenay Energy Resources, broughtin local distributor Bearings and Drives Ltd., who in turn put the ControlTechniques Drive Center in York, PA on the project. After surveying the cranes,their recommendation for improving the situation was the installation of tworegenerative crane control systems.

Each crane now regenerates the excess energy using a ControlTechniques' Unidrive SPM modular ac drive that operates in regenerativemode to feed power back into the plant's electrical supply. Another UnidriveSPM provides hoist control.

Each 150-hp, four-quadrant drive system gives substantialenergy savings of $10,000-12,000 per year with very low harmonic distortion(less than 4 percent) and maintains a near unity power factor throughout theentire speed range. In addition, because of the virtual elimination of heatgeneration, the need for air conditioning is eliminated, which produces anadditional energy savings of another $5,000 annually per crane.

Substantially reducing mechanical wear and tear andcontrolling the cranes' acceleration and deceleration means no more direct onlinestarting, and a substantial reduction in the need for maintenance. The costsavings across the board has provided very fast payback.

The Unidrive SP Modular drives range offers flexible powerfrom 60 to 2,900 hp on a common dc bus system with or without an active frontend (regenerative, four-quadrant operation). Very high current motors may becontrolled using a multi-drive modular system. The drives are configurable intofive operating modes including open and closed loop, vector, servo andregenerative modes. This range gives connectivity for most industry-standardnetworks and accepts 14 position feedback protocols. With a range of plug-inmodule options, its onboard PLC can be supplemented with a variety of programmablemodules.

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